Tubing and method of making coated tubing



May'Z, 1961 l. cAPLAN ErAL TUBING AND METHOD oF MAKING COATED TUBINGFiled sept; 4, 1958 s y mm@ w m GW whim Y@ 4v f f A may I, d Z d a l .n,wow/m@ M f iii- A wm H| |V @Sk RNQ NA QSS Tw* .wwwwvww LVV Q \\vb NMSQMMQQ f uw. LWN ik l G 0 Q Q Q D o C D D e D O O G o o o o o o o G NIT DNR n G A A Q G TA A Q s NNW .I 3f TY |L K TM. MMI .Nhkw lwvnu MASQ m35 HN Nmwww NAD MQ YN NN YN w the drawing,4 in which:

TUBING AND METHOD OF MAKING COATED TUBING Isadore Caplan, Bernard E.Frank, and Gerard T. Ruflin, Rochester, N.Y., andV Robert F. Thomson,vGrosse Pointe Woods, Mich., assgnors to General Motors Corporation,Detroit, Mich., a corporation of Delaware Filed Sept. 4, 1958, Ser. No.758,986

11 claims. (cl. 13s- 145) This `invention relates to the manufacture ofinteriorly coated welded steel tubing. A primary object of the inventionis to provide a method of making welded steel tubing Whilesimultaneously coating the inner surface thereof with a permanentcoating of an aluminum-zinc alloy which is securely bondedlto thesurface of the tubing.

The invention comprehends making steel tubing from a cleaned flat stripof steel which is continuously moved longitudinally through atube-forming mill. The tubeforming mill transversely bends the strip ofsteel into a tubular configuration. An alloy of aluminum and zinc isprogressively introduced into the tubing during its formation. Prior tothe complete formation of the tubing and about at the same point on the.tube-forming mill where the aluminum-zinc alloy is introduced into thetubing, the small diameter pipe which discharges a non-oxidizing gasenters the tubing and extends longitudinally therein. After the alloy isintroduced into the interior of the tubing the formation of the tubingis completed and the tubing is passed through a' suitable weldingdevice. It is subsequently passed through sizing means which impart thedesired finish cross-sectional configuration of the tubing.Y 'After thetubing has .been

sized, it is passed through a heating means where the` causes thevaporized alloy to condense `as a uniform coating on the inner surfaceof the tubing. If desired,

`the Vtubing can then be given asuitablek coating on its outer surface,resized, and cut to useful lengths age or coiled on reelsfor storage.

Further objects, features and advantages of the present invention willbecome more apparent through the followfor stora suitable cleaningcompound, a rinse 18, a pickling bath 20 and a subsequent rinse 22.After passing through the cleaning unit 14, the strip S is dried by anair blowoff 24 and directed into the tube-forming mill 12.

The tube-forming mill 12 is composed of a group of forming devices 26,each of which includes a pair 4of rolls 28 and 30. A sulicient number offorming devices 26 are included in the forming mill to impart thedesired circular cross-sectional configuration to the flat steel stripS. The forming devices progressively bend the flat steel strip S into atube T having an open seam Y 32 at the top where edges 34 and 36 of thestrip abut.

Prior to the complete formation of the tube T, a small diameter Vpipe'38 for conveying a non-oxidizing gasis introduced into the tubinginterior as shown more clearly in Figure 2. An aluminum-zinc alloy mayalso beintroduced into the tubing at this point as a wire 40 through thesmall diameter pipe 38. The formed tube T then passes between weldingrolls 42 or other suitable welding apparatus to close the seam 32 formedby the abutting edges 34 and 36 of the steel strip S. The formed tubethen passes between sizingrolls 44 which further impart desiredcross-sectional configuration of the tubing and then through a swagingdevice 46 for a final sizing operation. i

From the mill 12 the formed tubing T passes into a suitable heatingchamber 48 where the aluminum-zinc alloy which was placed in the tubingis vaporized. The

ing descripton of preferred embodiments thereof and from vaporized alloypenetrates the inner surface 50 of the tubing and forms alstrong alloybond therewith. The specific construction Yof the means by which thetubing is heated is not material to this invention. It may be of anyconstruction which will suciently raise the tubing to a temperaturewhich is suicient to vaporize the aluminum-zinc alloy and preferablysimultaneously anneal the tubing. Although the annealing operation cantake place after the vaporization operation, it is generally preferredto accomplish both simultaneously. Satisfactory results have beenobtained using annealing rolls such as are described in the- UnitedStates Patent No. 2,822,291, Rea

I. Hahn, patent February 4, 1958, and which is 'assigned to theassigneeof the present invention.

VFrom the heating chamber 48 the tubing T passes Vthrough a=coolingchamber S2Which serves to solidifythe vaporized alloy to'form a coating51 on the inner sur" can also bei'used. Whenrthe tubing is cooled inthis i Figure" l is a diagrammatic Vview showing a Vtubeform- 'i -in`g'n 1i1l provided with suitable apparatus for carrying out thepresentinvention; t

As shown' in Figure 1 the a dat steel strip S "of AISLlOlO, AISI` 1020,AISI 1008 `steel or'the like, which is coil'edon' a reel 10- positionedL adjacent one end of`a "tube-forming mill 12. Between.

is another-'enlarged sectional viewfalong the Y' line 5-5 of Figure l. l'-1 tubing T is formed frommanner, the alloy" coating 51 solidiessorapidly that there is substantially no tendency Vfor the coating metalto run toward the bottom of the tubing. lt has been Y observedthatthealloy coating` formed by this invention bottom of the'tubing with thethickness at the bottom being only negligiblyY greater. Y

A protective atmosphere is preferably "maintained around the outsideofthe tubing when it is in a heated condition 4to'prevent oxidationofthe outer surface of the I r'tubing-'and to insure against oxidationwithin the tubing. a Accordinglyfthe `heating and" cooling means areVprefer'- ably housed in the closed chambers 48 and 52 to facili-ytatethisfend.v After cooling, the V.tubing can then be is afclaning.unit 14 whichprepares the surface of the:

otating circular wire brushesl lwhiichiare liooded'with vresized, gifdesired, and" cut into'whatever lengthsar desired] lf cut into extremelylong lengths, the tubing is' coiled onto'long reels but it canYbecutinto short `lengths''and' stored in straiglit'A pieces, rifdesired.` `f1j Priorl to' enterig th`e Vtube'mill theflat steel Ystrip Sis preferably continuously cleaned lof Vall dirt,v steel, ox

des, oils or other materials by the cleaning unit 14, shown in thedrawing. The cleaning unit consists of two motor driven contra-rotatingwire brushes 16 which scrub the side of the strip which is to form theinner surface Si) of the tubing T. The brushes 16 of the cleaning unitare Hooded with a hot alkaline cleaning compound to carry away theloosened soil. One such alkaline cleaning solution which may be used isan aqueous solution which is saturated with sodium silicate and sodiumhydroxide. The steel strip then passes under a water spray or rinse 18to remove any cleaning compound which may be adhering to its surface.After rinsing, the steel strip can be passed through a pickling bath 20,such as an aqueous solution containing 30%, by volume, hydrochloricacid. After the pickling treatment, the strip is passed through therinse 22 to remove any traces of the pickling solution which may beadhering to its surface after the pickling treatment, and is thensubjected to an air blow-oft` 24 to dry it.

Other methods of cleaning, such as sand blasting, spray washing,electrolytic cleaning, etc.. may be used in some instances to preparethe steel strip for use in accordance with the invention. However, Wehave found that the hereinbefore described method of cleaning providesespecially beneficial results when used in combination with the coatingmethod of the invention. The internal coatings applied to tubingsurfaces which are cleaned in this manner are particularly more adherentand permanent.

The coating metal can be introduced into the interior of the hollow andannular cylindrical tubing T in any convenient form. Our invention isespecially more conveniently practiced, however, if the coating metal isintroduced into the tubing in a wire form 40, as shown in the drawing.Generally it is preferred to introduce the coating metal into the tubingimmediately before it is fully formed. The wire 40 may be coiled on aroll 54 mounted on the tube-forming mill 12 and fed through a directingtube 56 into the interior of the tubing by means of a pair of drivingrolls S8. As shown in the drawing, the aluminum-zinc alloy wire 40 canbe introduced within the tubing through the small diameter pipe 38 whichis used to introduce the protective atmospherel into the interior of thetubing. The specic construction of this mechanism is not aipartvof thisinvention and any suitable device can be used to introduce the'coatingmetal at a substantially uniform rate. The particular rate at which themetal is introduced is variable to some extent and is governed by thesize and speed of movement of the tubing being formed.

As shown in United States Patent No. 2,771,669, filed in the names of J.W Armstrong, R. W. Spears and R. D. Williams and which is assigned tothe assignee of the present invention, particles of metal such as zinccan also be uniformly introduced into the interior of the tubingemploying a suitable hopper or funnel having a narrow outlet whichextends into the tubing prior to its complete formation.

Obviously, in order to provide a uniform coating, the amount ofaluminum-zinc alloy which is introduced into the interior of the tubingmust vary inl direct proportion with both the size and speed ofmovementv of the tubing which is to be alloy coated. Moreover, coatingcompositions of varying proportions can be produced by varying theamount of coating material introduced into the interior of the tubing.In general, highly satisfactory As a specific example, steel tubinghaving an outer diameter of approximately "ta inch and a wall thicknessof approximately 0.028 inch can .be successfully coated using analuminum-zinc alloy wire, approximately 0.032 inch in diameter, which ismoving at the speed of the tubing being formed. Similarly one can use analuminumzinc wire, approximately 0.047 inch in diameter, which is movingat the speed of the tubing to coat tubing having an outer diameter ofapproximately inch and a wall thickness of about 0.035 inch. In general,highly satisfactory results are obtained when a weight of approximately0.0025 pound per-foot of tubing and 0.0053 pound per foot of tubing are,respectively, used.

A non-oxidizing gas from a suitable reservoir (not shown) passes throughthe small diameter pipe 38 which is introduced into the tubing atapproximately the same point on the tube-forming mill where the coatingmetal is introduced into the tubing. As shown in the drawing, this smalldiameter pipe 38 has a part (not shown) which extends axially within theinterior of the tubing towards the swaging device to a point where thetubing is completely formed. The discharge end of the pipe is located ata point where the tubing is completely formed so that the reducingatmosphere emitted therefrom will be substantially contained within theinterior of the tubing. Thus, a protective atmosphere is establishedwithin the tubing as it moves along the various following operations.

It is also desirable to employ a protective atmosphere outside thetubing in the operation following tube formation. Heating the tubing inan oxidizing atmosphere may cause a deleterious corrosion and spallingof the outer surface which is detrimental to subsequent outer coatingoperations. Thus, as shown in Figure l, the heating and cooling of thetubing can be done within closed chambers in which a protectiveatmosphere is established. The non-oxidizing gas can be introduced intothe chambers by means of additional piping from the source of supply(not shown).

A gas which protects the interior and exterior of the tubing ispreferably 20% to 25% reducing in nature. For example, highlysatisfactory results are obtained with a gas which has the followinganalysis: 10% carbon monoxide, 18% hydrogen, 41/2 carbon dioxide, 1%methane and the balance nitrogen, all proportions by volume. However,substantially pure mixtures of hydrogen, carbon monoxide, nitrogen,helium, argon, can also be used.

From the swaging device or sizing device the tubing passes into theheating chamber where it is heated above the vaporization temperature ofthe aluminum-zinc alloy which was deposited within the tubing during itsformation. The vaporization of the aluminum-zinc alloy permits the alloyto penetrate the inner surface of the tubing to some extent to producean exceedingly uniform tenacious alloy coating. The vaporization of thealloy and the annealing of the steel tubing can be accomplishedsimultaneously, if desired, and in such instance the tubing should beheated to a temperature which is not only sufin thickness. InV mostinstances, the coating of approximately 0.001 inch. in thickness ispreferred.

ficient to vaporize the zinc but also to anneal the tubing. It has beenfound that satisfactory alloy coatings are obtained when the tubing isheated to a temperature beween 1700 F. and 2400 F. In general, highlysatisfactory annealing and coating results are obtainable in thetemperature range of approximately 2000 F, to 2400" F.

After the tubing has been cooled, if desired, it may be coated on itsouter diameter and subsequently passed Vthrough a pull-out unit toresize the outside of the tubing.

In some instances the various coating and heat treatments to which thetubing is subjected may affect the outer dimensions of the tubing sothat resizing is required. Should resizing be required, the tubing canbe passed through a pull-out unit which is, in general, similar to thefinal sizingdevices employed on the tube mill which g pose the basemetal.

l the surface of the steel.

the -white corrosion products of a pure zinc coating which would spallor flake from the surface and subsequently deleteriously aiiect the fuelsystem. Moreover, steel tubing which isused to convey especiallycorrosive materials must-be `correspondinglyV of especially highcorrosion resistance.

It has been foundthat when 'vaporizing pure zinc metal in accordancewith the method` set forth in the aforementioned Armstrong et al.patent, `the zincmetal forms dendritic structures on the inner surfaceof the tubing with-pure base `metal exposed therebetween. Thus, thecoating formed, 1unlike an immersion coating, is

somewhat discontinuous ands contains pinholes at which corrosion canoccur. The galvanic or sacrificial properties of the zinc will protectthese bare spots to some extent but yover extended periods of time thecorrosion resistance of the coating is seriously aected.

Moreover, due to the dendritic structure and the formation offerro-zinclintermetallic compounds, the pure zinc coating tends to beundesirably brittle/for some applications. Where the tubing is subjectedto an exces- :sive amount of bending, the harder and more brittle zinccoating is not Vsufficiently ductile to resist cracking and delaminationand therefore exhibits especially poor cor' rosion resistance.

It has now been found thatby adding rather large, amountsxof aluminumto-zinc an especially satisfactory coating can be formed that is bright,ductile and more corrosion resistant. alloy wire onto the inner surfaceof the tubing no dendriticstructure is formed and` an exceptionallysmoother, uniform, bright and shiny coating isy obtained. Sincesubstantially no dendritic structure isV present in the aluminum-zinccoating, there are no pinholes present to ex- Moreover, the laddition ofthe aluminum minimizes the formation of the brittle ferro-zincintermetalliccompounds which contribute to the brittlenessof thecoating.

A further advantage of the coating formed with the aluminum-zinc alloyin the method hereinbefore vde-` scribed is that it contains a thinoverlay of an aluminumrich alloy which virtu-ally eliminates the white`corrosion products common to a pure zinc coating This thin aluminum-richoverlay provides the coating with anexceptionally higher corrosionresistance than'that even obtained under a hot dip process employing asimilar aluminumainc alloy. f In addition to the more ductile .andcontinuous characteristics of the coating, Vthis invention furtherprovides corrosion protection by means of its ne sacrificial properties.Y

1The ductile, continuous, highly corrosion-resistant` will not even wetthe surface onto which it is to be deposited and that it merely meltswithout evenalloying to We have Vfound especially satisfactory results'are obtainablewhen forming a coating in accord-ance with thev presentinvention'with a zinc base alloy containing, by weight, about 10% to 20%alu- /minum and 80% -to 90%V zinc. 'H owever, in some instances, it ispreferable to employ a zinc base alloy con- By vaporizinganaluminum-zinc weight, and as high as 30%, by weight. in general,highly satisfactory coatings can be obtained employing a zinc base alloywire containing approximately 18%, by weight, aluminum and the balancezinc. 3

We have also found that highly satisfactory coatings are obtained with asodium-aluminum-zinc alloy in which the sodium content is about 0.08% to0.12%, by Weight. The addition of the sodium increases the fluidityofthe aluminum-zinc alloy and contributes to the formation of anexceptionally adherent and more ductile coating. Y In general, we haveVfound that zinc base alloys containing 0.08% to 0.12%, by Weight,sodium and about 12% to 18%, by weight, aluminum and the balance zincwill provide highly satisfactory results. In some instances,

however, satisfactory coatings may be formed With'the abovementionedYamount of sodium in zinc base alloys containing as low as 5%, byweight, aluminum andas high as 30%, by weight, aluminum. f

We have found that sodium in combinationV with Yaluminum and zincprovides a highly satisfactory smooth, brighttand ductile alloy coating.However, in some instances alloys containing another alkali metal,particularly lithium and-potassium, may also be used in place of aportion or even the entirety of sodium in the alloy described above toobtain highly satisfactory vaporized coatings. Moreover, it isalsocontemplated that in certain instances the alkaline earth metals,particularly calcium, can be used in place o f a portion or all of thesodium in the aluminum-sodium-zinc alloy described above.

It is to be understood that the term abutment is used herein in itsprimary meaning; i.e. indicating touching or contacting. Thus,overlapping edges of a lap seam is also comprehended within the phraseabutting edges as well as non-overlapping contacting edges, such asshown in the drawing. Our invention therefore encompasses making tubinghaving `abutting edges from a metal strip having scarfed longitudinaledges, the surfaces thereof being non-perpendicular to the major surfaceof the steel strip, as well as from a steel strip having itslongitudinal edges perpendicular to the major surface of the strip;

` It is to be'further understood that the term zinc base alloys asusedherein, refers to those alloys which contain zinc in greater proportionsthan 50%, by weight.

Although our invention has been described in connecf tion with singlewall tubing and other certain specific ex-V an adherent, uniform alloycoating during formation of'said tubing from a steel strip, said methodcomprising the steps of longitudinally moving a substantially'at steelstrip while continuously transversely bending it into a tubularconfiguration having Yabutting edges forming a seam, introducing analloy containing aluminum and vzinc into the interior of the tubing,after it is completely formed heating the tubing to a degree sufficientto vapor ize said alloy within the tubing and thereafter cooling thetubing sufficiently to cause said alloy to solidify so as to form asmooth and ductile alloy coating on the interior of the tubing. l

2. The method of coating the interior of steel tubing with an adherent,uniform alloy coating during formation 'of said tubing from a steelstrip, said method comprising the steps of longitudinally moving a steelstrip While continuously transversely bending it into aV tubularconfiguration having abutting edges forming a seam, introducing a `zincbaseall'oy containing approximately y 57% to 30% by weight aluminum intothe interior of the tubing, after it is completely formed heating thetubing ,to agdegree suicient to vaporize said alloy within the tubingand thereafter cooling the tubing suiciently to cause said alloy tosolidify so as to form a smooth and ductile alloy coating on theinterior of the tubing.

with an adherent, uniform alloy coating during formation of said tubingfrom a steel strip, said method comprising the steps of longitudinallymoving a steel strip while continuously transversely bending it into atubular configuration having abutting edges forming a seam, introducingan alloy containing about to 30% by weight of aluminum and 70% to 95% byweight of zinc into the interior of the tubing, after it is completelyformed heating the tubing to a temperature of approximately 1700 F. to2400 F. to vaporize said alloy within the tubing, and thereafter coolingthe tubing sufficiently to cause said alloy to solidify so as to form asmooth and ductile alloy coating on the interior of the tubing.

y 4. A method of coating the interior of steel tubing with an adherent,uniform alloy coating during formation of said tubing from a steelstrip, said method comprising the steps of longitudinally moving a steelstrip while continuously transversely bending it into a tubularconfiguration having abutting edges forming a seam, introducing a zincbase alloy containing aluminum and a metal from the class consisting ofalkali and alkaline earth metals into the interior of the tubing, afterit is completely formed heating the tubing to a degree suflcient tovaporize said alloy within the tubing, and thereafter cooling the tubingsuiciently to cause the alloy to solidify so as to form a smooth andductile alloy coating on the interior of the tubing.

5. The method of coating the interior of steel tubing with an adherent,uniform alloy coating during formation of said tubing from a steelstrip, said method comprising the steps of longitudinally moving a steelstrip while continuously transversely bending it into a tubularconfiguration having abutting edges forming a seam, introducing an alloycontaining, by weight, about 12% to 18% aluminum, about 0.08% to 0.12%sodium and the balance substantially zinc into the interior of thetubing, after it is completely formed heating the tubing to atemperature of approximately 1700" F. to 2400 F. to vaporize said alloywithin the tubing, and thereafter cooling the tubing suciently to causethe alloy to solidify so as to form a smooth and ductile coating on theinterior of the tubing.

6. The method of coating the interior of steel tubing with an adherent,uniform alloy coating during formation of said tubing from asubstantially flat steel strip, said method comprising the steps oflongituldinally moving a substantially flat steel strip whilecontinuously transversely bending it into a tubular configuration havingabutting edges forming a seam, continuously introducing a wire of analloy containing about 5% to 30%, by weight, aluminum and about to 95%,by weight, zinc into the interior of said tubing prior to the completeshaping thereof, discharging a non-oxidizing gas into the interior ofthe tubing at a point where it is substantially completely formed, afterthe tubing is completely formed heating the tubing to a temperature ofabout 1700 F. to 2400 F. to vaporize said alloy within the tubing, andthereafter cooling the tubing to cause said alloy to solidify so as toform a smooth and ductile coating on the interior of said tubing.

7. An article of manufacture comprising a steel tube having an innersurface, a coating on said inner surface of a vaporized zinc base alloycontaining approximately 5% to 30% by weight of aluminum.

8. An article of manufacture comprising a steel tube having an innersurface, a coating on said inner surface of a vaporized alloy consistingessentially of approximately 5% to 30% by weight of aluminum and about70% to 95 by weight of zinc.

9. An article of manufacture having a smooth and ductile zinc base alloycoating thereon comprising a steel tube and an interior coating on saidtube of a vaporized alloy containing, by weight, about 5% to 30%aluminum and about 70% to 95% zinc, said coating being especially highin aluminum content at its outer surface.

10. An article of manufacture having a smooth ductile zinc base alloycoating thereon comprising a steel tube and an interior coating on saidtube of a vaporized alloy containing, by weight, about 5% to 30%aluminum, about 70% to 95% zinc and about 0.08% to 0.12% sodium, saidcoating being especially high in aluminum content at its outer surface.

1l. An article of manufacture having a smooth ductile zinc base alloycoating thereon comprising a steel tube and an interior coating on saidtube of a vaporized alloy containing, by weight, about 5% to 30%aluminum, about 70% to 95% zinc and about 0.08% to 0.12% of a metal fromthe class consisting of alkali and alkaline earth metals, said coatingbeing especially high in aluminum content at its outer surface.

References Cited in the le of this patent UNITED STATES PATENTS2,771,669 Armstrong Nov. w27, 1956

